(a) Technical Field of the Invention
The present invention generally relates to electro-mechanical switches, more particularly to micro-electromechanical switches (MEMS), and most particularly to high power RF MEMS.
(b) Description of Related Art
In communications applications, switches are often designed with semiconductor elements such as transistors or pin diodes. At microwave frequencies, however, these devices suffer from several shortcomings. Pin diodes and transistors typically have an insertion loss greater than 1 dB, which is the loss across the switch when the switch is closed. Transistors operating at microwave frequencies tend to have an isolation value less than 20 dB. This allows a signal to xe2x80x98bleedxe2x80x99 across the switch even when the switch is open. Pin diodes and transistors have a limited frequency response and typically only respond to frequencies below 20 GHz. In addition, the insertion losses and high isolation value for these switches vary depending on the frequency of the signal passing through the switches. These characteristics make semiconductor transistors and pin diodes a poor choice for switches in microwave applications.
U.S. Pat. No. 5,121,089, to Larson, disclosed a new class of microwave switchxe2x80x94the micro-electro-mechanical (MEM) switch. The MEM switch has a very low insertion loss (less than 0.2 dB at 45 GHz) and a high isolation when open (greater than 30 dB). In addition, the switch has a large frequency response and a large bandwidth compared to semiconductor transistors and pin diodes. These characteristics give the MEM switch the potential to replace traditional narrow-bandwidth PIN diodes and transistor switches in microwave circuits.
The Larson MEM switch utilizes an armature design. One end of a metal armature is affixed to an output line, and the other end of the armature rests above an input line. The armature is electrically isolated from the input line when the switch is in an open position. When a voltage is applied to an electrode below the armature, the armature is pulled downward and contacts the input line. This creates a conducting path between the input line and the output line through the metal armature.
Micro-electromechanical switches of the general type described above are, however, prone to premature failure. The cause of the premature failure is linked to the damage resulting from the impact of the armature contact with the substrate contact. This damage is exacerbated by the fact that conventional MEM switches have armature contacts that impinge on the substrate contact surface at an angle. The angled impact results in all the impact energy being transferred to a relatively small area, thereby ultimately causing premature failure due to both increased impact per unit area and heat caused by resistive heating due to increased current density through the small area of actual contact.
The present invention solves this and other problems by providing a torsion spring which is configured to result in a substantially conformal contact between the contact plates. The resultant MEMS has increased durability, and because the substantially conformal contact results in a better electrical contact, there is less heating and the MEM switch can handle more power. Thus, a more durable and versatile MEM switch results.
One embodiment of the present invention provides an armature mounted torsion spring, wherein the torsion spring is configured to provide sufficient flexibility such that when an armature electrode is electromechanically brought into contact with a substrate electrode the electrodes provide substantial conformity with one another and thus maximize contact area, reduce wear and reduce Ohmic resistance.
This invention provides a new RF MEM switch in which the RF line is loaded with a torsion spring to achieve a conformal metal to metal contact. A conformal metal to metal contact assures a maximum contact area and lowest contact resistance, and, therefore, provides for critical long term reliability and good heat dissipation thus allowing for improved high-power handling.
In another embodiment, the present invention provides a torsion spring for an electromechanical switch. The torsion spring comprises a set of tines with the set of tines having at least one tine. The set of tines extends from a free end of the armature of the switch, and includes a terminus portion rotatably suspended between the tines. The terminus portion includes a conducting transmission line, at least a portion of which is exposed for electrical contact. The conducting transmission line have a length selected such that the exposed portion of the transmission line forms a circuit between the input and output of the micro-electro-mechanical switch when the micro-electro-mechanical switch is urged into a closed position. When the switch closes, the terminus portion rotates via the tines to form a conformal connection between the exposed portion of the conducting transmission line and the input and output of the switch, thus optimizing the electrical flow therebetween.
In another embodiment of the torsion spring, the portion of the conducting transmission line exposed for electrical contact is in the form of a plurality of dimples. Each dimple corresponds to the contact to be made with the input and the output, respectively. The dimples combined with the rotatable nature of the terminus provide a conformal contact between the conducting transmission line and the input and the output to form a circuit therebetween.
The torsion spring is preferably constructed of a material selected from a group consisting of silicon nitride, Type III-V semiconductor materials, and silicon dioxide. The conducting transmission line is preferably formed from a titanium adhesive layer and a gold conductor layer.
The set of tines of the torsion spring preferably includes a plurality of tines, and more preferably includes two tines.
In another embodiment, the present invention provides a micro-electro-mechanical switch comprising a substrate with an input line, an output line, and a substrate electrode formed on the top of the substrate, all separated from each other. The switch further includes an armature having a first beam structural layer with a first end mechanically connected with the substrate and a second end including a set of tines with at least one tine. A terminus portion is suspended between the tines, and includes a conducting transmission line positioned over the input and output lines. At least a portion of the conducting transmission line is exposed for conformal contact with the input and output lines. The armature further includes an armature electrode positioned directly above the substrate electrode and suspended on the armature. An insulating layer is positioned between the armature electrode and the substrate electrode to prevent short-circuiting therebetween. Thus, when the switch is actuated into a xe2x80x9cclosedxe2x80x9d position, the terminus is free to rotate to ensure a conformal contact between the exposed portion of the conducting transmission line and the input and output lines in order to form a circuit therebetween to permit the flow of electricity.
The armature of the switch is preferably modified with the same enhancements discussed relative to the torsion spring embodiment above.
In another embodiment of the micro-electro-mechanical switch, the insulating layer is formed as a second beam structural layer under the armature electrode. The first and the second beam structural layers are formed of materials selected such that their mechanical and thermal properties provide a desired amount of bowing when the switch is activated.
Another embodiment of the present invention provides an armature for a micro-electro-mechanical switch having a torsion spring. The armature comprises a first beam structural layer having a first end for mechanically connecting with a substrate of a micro-electro-mechanical switch and a second end including a set of tines including at least one tine. A terminus portion is suspended between the tines, and includes a conducting transmission line configured to be positioned over the input and output lines of a micro-electro-mechanical switch. At least a portion of the conducting transmission line is exposed for conformal contact with the input and output lines. An armature electrode is directly above a substrate electrode of the micro-electro-mechanical switch and suspended on the armature, and an insulating layer is positioned between the armature electrode and the substrate electrode to prevent short-circuiting therebetween when the armature is assembled in a micro-electro-mechanical switch and actuated into a closed position. When the armature is assembled in a micro-electro-mechanical switch and is actuated into a xe2x80x9cclosed xe2x80x9d position, the terminus is free to rotate to ensure a conformal contact between the exposed portion of the conducting transmission line and the input and output lines in order to form a circuit therebetween to permit the flow of electricity.
The armature is preferably modified with the same enhancements discussed relative to the torsion spring embodiment above.
In another embodiment of the armature, the insulating layer is formed as a second beam structural layer under the armature electrode, with the first and the second beam structural layers formed of materials selected such that their mechanical and thermal properties provide a desired amount of bowing when the switch is activated.